As a brake for a vehicle, there is a conventionally well-known braking device which achieves braking by bringing a disc, provided at a rotating wheel side, and a plate, provided at an axle side, into pressure contact with each other by means of a piston operated by brake oil. Such a braking device is normally provided with a brake abrasion detecting device for detecting a time of abrasion of a plate and a disc, to facilitate an inspecting operation of the brake, and to prevent a failure from occurring in the brake.
As such a brake abrasion detecting device, there is a device shown in, for example, FIGS. 9 and 10, which is an example of a wet type multiple disc brake 1. It has a hollow outer gear 2, having an almost cylindrical shape on the outer perimeter portion thereof. The inner surface of the hollow body of the outer gear 2 is provided with spline grooves. The hollow body houses therein a number of annular plates 3, having outer perimeter portions which are engaged in the spline grooves on the inner surface of the outer gear 2, and annular discs 4, which are alternately provided between these plates 3. Spline grooves are formed on an outer perimeter surface of an inner gear 5, having an almost cylindrical shape, which is provided at the axle side; and the inner perimeter portion of each of the discs 4 is engaged in a spline groove on the outer perimeter portion of the inner gear 5. Linings (not illustrated) are provided on both end surfaces of each of the discs 4, which face the plates 3. An annular cylinder 2a, having a radius of the same size as those of the plates 3 and the discs 4, is formed in one end of the outer gear 2. An annular piston 6 is provided inside the cylinder 2a. Further, brake oil is scaled by means of seal members 7 on the inner perimeter surface and the outer perimeter surface of the piston 6.
When brake oil is supplied from a supply line (not illustrated) into the cylinder 2a, the piston 6 is displaced toward the plates 3 and the discs 4. This displacement brings the plates 3 and the discs 4 into pressure contact with each other between the piston 6 and the other end of the outer gear 2. Thereby, braking force is applied to the inner gear 5, and the rotation of the wheel is braked.
A brake abrasion detecting mechanism 40 is attached at the end of the outer wall of the outer gear 2 behind the piston 6 (on the left in FIG. 10). The brake abrasion detecting mechanism 40 has a fixed member 35 of which one end is screwed into the end surface of the outer gear 2. A portion of the fixed member 35, which is inside the outer gear 2, is hollow. The fixed member 35 has a shaft 31 which penetrates through this hollow portion, and the shaft 31 penetrates through a hole 2b which is provided in the outer wall at the end of the outer gear 2. A seal member 37 is provided in the vicinity of the entrance of the hole 2b at the plate 3 side. A ring 32 is fixedly provided at a predetermined position on the outer perimeter portion of the shaft 31 inside the aforesaid hollow portion of the fixed member 35, and a spring 33 is provided between the ring 32 and an end face 34 of the aforesaid hollow portion. The spring 33 is always given momentum in a direction in which it stretches. The front end portion of the shaft 31 is pressed against a rear end surface of a pressurizing portion 6a of the piston 6 by the momentum of the spring 33.
A hole 38, through which the shaft 31 penetrates, and a hole 39, in the vicinity of the hole 38, are provided at the rear portion of the hollow portion of the fixed member 35. An annular magnet 41 is attached on the outer perimeter of the rear end portion of the shaft 31 penetrating through the hole 38 so as to move inside the hole 38 following the movement of the piston 6. A reed switch 42 is provided at a predetermined location in the hole 39, and when the magnet 41 approaches within a predetermined distance of that location, the output power contact point of the reed switch 42 is actuated. The output power contact point is inputted into a computing unit (not illustrated).
In a brake abrasion detecting device with the configuration as in the above, when the brake is applied by stepping on a brake pedal (not illustrated), the piston 6 moves forwardly, and brings the plates 3 and discs 4 into pressure contact with each other. Following this forward movement, the shaft 31 is moved as far as the same distance by the momentum of the spring 33. When the brake is released, back pressure is applied to the piston 6 by the oil pressure of brake cooling oil, which is supplied to the plates 3 and the discs 4. Then the piston 6 is returned as far as a predetermined distance by means of a piston returning amount adjusting mechanism (not illustrated), and the shaft 31 is returned against the momentum given thereto. In this way, while the amount of abrasion wear of the plates 3 and the discs 4 is small, the magnet 41 of the shaft 31 is not at the position to actuate the reed switch 42, even if the piston 6 is moved to the forward end. When the amount of abrasion wear of the plates 3 and the discs 4 is greater than the allowable value and the piston 6 reaches the forward end, the reed switch 42 is actuated by the magnet 41, and the time of abrasion can be detected.
However, in the aforesaid conventional braking device 1, there are a wide range of variations in the thickness of the plates 3 and the discs 4, the thickness of the pressurizing portion 6a of the piston 6, the length of the shaft 31, or the like, and a wide range of errors in the dimensions, of these products after being assembled. Due to these variations and errors, the conventional brake abrasion detecting device often makes erroneous detection. For this reason, it often happens that when the brake abrasion detecting device determines that abrasion has just occurred, that abrasion has not yet occurred, or that abrasion has advanced more than an allowable value. Accordingly, since the precision in abrasion detection is not stable, an operation of confirming the degree of the abrasion by disassembling the brake cannot be omitted; therefore there is a disadvantage of poor workability when performing maintenance.